A boosted air system (e.g., turbocharger, supercharger, etc.), as applied to an internal combustion engine, typically introduces noise. For example, a turbocharger's compressor and/or turbine blades may generate whining noises. Such disturbances may decrease longevity of a boosted air system or other components. In addition, such disturbances may subjectively annoy people and/or animals in proximity to an operating boosted air system.
In general, noise occurs as a result of component vibrations and/or aerodynamics (e.g., acoustics). Noise associated with component vibrations may originate from various sources such as bearings. For example, bearings can experience instabilities known as “whirl”. In contrast, acoustic noise typically originates from pressure fluctuations, which travel as longitudinal waves through air and/or other media.
Acoustic noise can be particularly noticeable in a turbocharger turbine that uses a variable geometry mechanism to control flow to the turbine wheel. In particular, substantial noise generation can occur due to interactions between variable geometry vanes and rotating turbine blades. Such interactions generate noise at what is commonly known as the blade pass frequency. The blade pass frequency noise is often high enough to generate customer complaints; thus, a need exists to minimize such noise.